Recovery of uranium



RECOVERY or on Application February 19, 1953, Serial No. 337,747

6 Claims. ((11. 204-90) This invention relates to the recovery of uranium from uranium-bearing ores. More particularly it relates to improvements in the acid leaching of such ores and the recovery of uranium from the leach liquor.

In the sulfuric acid leaching of uranium-bearing ores, the uranium is extracted as a complex acid in which the uranium has a valence of plus six. In order to convert all the uranium in the ore to the plus six state, present practice is to add an oxidizing agent, such as manganese dioxide, to the ore pulp during or prior to leaching. In separating the uranium from the leach liquor one practice is to reduce the uranium to the plus four state and adjust the pH to cause the uranium to precipitate as a uranous phosphate. If necessary, a phosphate to provide phosphate ion is added. This process requires substantial quantities of acid for leaching and of oxidizing and reducing agents for changing the valence of the uranium.

The object of this invention is to provide an improved process for the acid leaching of uranium-bearing ores and of recovering uranium from the leach liquor. A further object is to provide a process for the acid-leaching of uranium-bearing ores and for the recovery of uranium from the leach liquor which reduces to a minimum the quantities of chemical agents consumed.

The present invention accomplishes these objects by carrying out one or both of the extraction and precipitation steps in an electrolysis cell in which an anode compartment is separated from a cathode compartment by an anion exchange membrane. The term anion exchange membrane is herein used to designate a membrane through which anions will readily pass but which resists the transport of cations. Membranes having this quality are described in copending application Serial No. 205,413, filed January 10, 1951, now U. S. Patent No. 2,681,319, and may be prepared by milling particles of an anion exchange resin into certain polymeric materials particularly polyethylene.

The drawings illustrate diagrammatically a cell suitable for the practice of the invention. Fig. 1 represents a side elevation of the cell through line 1-1 of Fig. 2. Fig. 2 represents a front elevation of the cell through line 22 of Fig. 1. Fig. 3 represents a plan view through line 33 of Fig. 1. The cell is formed of two half-sections 4 and 5 between which is located the anion exchange 2,741,589 Patented Apr. 10, 1956 membrane 6. On each side of the membrane and in close proximity to it are the electrodes 7 and 8. Graphite or lead may be used for the electrodes. Graphite is preferred for the anodes 7 and lead for the cathodes 8. The electrodes are formed so as to permit the ready circulation of electrolyte around them. Each section of the cell is provided with means (not shown) for circulating the material in it. Cyclone mixers may be used for this purpose.

For the precipitation of uranium from the pregnant leach liquor, the liquor is added to the cathode section of the cell with an electrolyte preferably one containing sulfuric acidin the anode section. During electrolysis the sulfate ions of the leach liquor pass through the anion exchange membrane to the anode section. As each sulfate ion migratesthrough the membrane, two hydrogen ions are discharged as hydrogen gas at the cathode resulting in an increase of the pH of the catholyte. .Simultaneously with the discharge of the two hydrogen ions at the cathode two hydrogen ions are formed at'the anode through the decomposition of water resulting in a decrease in the pH of the anolyte. The net effect is the transfer of sulfuric acid from the cathode section to the anode section. At the same time the uranium in the cathode section is reduced from a valence of six to a valence of four according to the equation Since some phosphate ionis present in the leach liquor, a precipitate of uranous phosphate forms when the pH reaches 1.5 to 2. If there is vanadium present in the leach liquor, it will also be reduced and some will precipitate as V(OH)3. The precipitate may also be contaminated with iron and aluminum.

The quality of the precipitate can be substantially upgraded by redissolving it in pregnant leach liquor and subjecting the enriched liquor to electrolysis. This may be repeated several times and the leach liquor substantially enriched beforeit is subjected to electrolysis. If desired, this enriched liquor may be electrolyzed in stages with only a part of the uranium precipitated in each stage. The richest fraction may then be withdrawn and the balance redissolved in pregnant leach liquor.

According to the equation given above for the reduction of the uranyl group, the presence of hydrogen ion is necessary. In practice it has been found that the rate of reduction is adversely aifected by increasing pH. It is, therefore, desirable at the start of the electrolysis to have considerable residual acid in the leach liquor. With the leach liquor at pH 0.5 or lower at the start of the electrolysis, no difliculty was encountered in effecting complete precipitation of uranium before the pH rose to 2.0. Small amountsof uranium showed up in the filtrate after the electrolysis when a leach liquor of pH 0.64 was treated.

The following table gives data on a number of representative runs on a leach'liquor that also contained substantial amounts of vanadium, iron, and aluminum:

avenues TABLE I Batch electrolyses of pregnant carno tite liquors Orig. Assay Final Assay P GPpt.

errade Time Av. Vol kwh.

Number cent Amp. Per- U30is p U308 p Rwy (muL) Volt (1.) #UaOs cent g./1. g./1. U30:

Average 0. 030 99. 2 32 18 From these data it can be observed that the quality of the filter cake depends to some degree upon the initial concentration of uranium in the leach liquor. This was confirmed by tests in which a filter cake containing 10% U308 was dissolved in a volume of leach liquor equal to the volume from which it was obtained, the enriched liquor electrolyzed, and the dissolution and electrolysis repeated a second time. in this manner the filter cake was upgraded to 27% U308 and the recovery of uranium in each electrolysis was better than 99.5%.

While the precipitation of uranium is being accomplished in the cathode section of the electrolysis cell, the leaching of the ore may be carried out in the anode section. As noted above, acid is continuously formed in the anode section during the electrolysis. This acid is useful for the extraction of uranium from the ore and by having this extraction take place in the anode section of the cell a number of advantages are obtained. The oxidative conditions that exist in the anode section will serve the function of the usual oxidizing agents employed in leaching. Furthermore, the ampere efliciency of the cell will be increased by the ore neutralizing some of the acid as it is formed.

The following example will illustrate the ore extraction operation:

A slurry consisting of 4 liters of 1 N H2804 per 3 kilograms of ore that had been ground to pass a 60 mesh sieve was used as anolyte while the cell was operating as heretofore described for the precipitation of uranous phosphate from leach liquor. The ore analyzed 0.39% U308. When the leach liquor electrolysis was complete (pH of 2 in cathode section), the anolyte was drained 011 and filtered. The residual ore then contained 0.07% UsOs and the filtrate analyzed 3.3 g./l. UsOs.

This invention contemplates the independent practice of the extraction and precipitation steps as well as their being simultaneously carried out at the opposite electrodes. Thus, there will be circumstances when it will be preferable to use the electrolysis cell only for'the precipitation of uranium and to use the anode section only for the recovery of acid which may be used in the conventional extraction of uranium or for other purposes. Similarly there will be circumstances in which it is desirable to carry out only the extraction step and use a spent acid or even a salt solution in the cathode section. In some circumstances a balanced production may require a combination of the foregoing or that one or the other of the steps be partially performed in the electrolysis cell and finished outside of it. For instance, if the extraction is incomplete when the pH of the cathode has reached the desired point, it may be completed outside the cell by the current practice or an acid or salt solution may be used to replace the finished leach liquor in the cathode this electrolysis graphite cathodes are preferable to lead.

The following table summarizes the results of a number of vanadium recovery runs performed precisely the same as has been described for uranium precipitation:

TABLE 11 Original Assay Final Assay P t ercen Run V O ait? and; W332 5 3 pH V205 pH fade) Graphite cathodes employed.

These data also show that enriched leach liquors give higher grade precipitates.

I claim:

l. The process of recovering uranium from uraniumbearing ores which comprises subjecting the finely ground ore to the leaching action of sulfuric acid in the anode section of an'electrolysis cell having an anion exchange membrane separating the anode section from the cathode section, simultaneously electrolyzing pregnant leach liquor which contains phosphate ion in the cathode section, continuing the electrolysis until the uranium is precipitated as uranous phosphate, separating the uranous phosphate from the barren leach liquor, separating the leached ore from the new pregnant leach liquor, and with the phosphate ion added, if necessary, electrolyzing said new pregnant leach liquor in the cathode section of a cell having an anion exchange membrane separating the anode and cathode sections while a new batch of ore isextracted in the anode section.

2. The process of claim 1 in which the electrolysis is continued until the pH of the catholyte reaches 1.5 to 2.0.

3. The process of claim 2 in which the pregnant leach liquor at the start of the electrolysis has a pHbelow 0.64.

4. In the process of recovering uranium, the improvement which comprises electrolyzing pregnant leach liquor containing dissolved uranium and phosphate ion and sulfate ion in the cathode section of an electrolysis cell having an anion exchange membrane separating the cathode section from the anode section, wherein the anolyte contains sulfate ions, until the pH of said leach liquor reaches 1.5 to 2.0 whereby uranous phosphate is precipitated.

5. The process of claim 4 wherein the pregnant leach liquor containing dissolved uranium is enriched by hav ing dissolved in it precipitated uranous phosphate from a previous electrolysis.

6. In the process of recovering uranium and vanadium values, the improvement which comprises electrolyzing pregnant leach liquor, which contains dissolved uranium and vanadium as well as phosphate and sulfate ions, in the cathode section of an electrolysis cell having an anion exchange membrane separating said cathode section from an anode section wherein the anolyte contains sulfate References Cited in the file of this patent UNITED STATES PATENTS 1,382,808 Sem June 28, 1921 1,403,463 Allingham Jan. 10, 1922 1,448,036 Pearson et a1 Mar. 13, 1923 1,462,421 Pearson et a1 July 17, 1923 2,636,851 Juda et a1 Apr. 28, 1953 OTHER REFERENCES The Chemistry of Uranium, Part 1, Joseph J. Katz, 1st ed., McGraw-Hill Book Co. (1951), page 111. 

1. THE PROCESS OF RECOVERING URANIUM FROM URANIUMBEARING ORES WHICH COMPRISES SUBJECTING THE FINELY GROUND ORE TO THE LEACHING ACTION OF SULFURIC ACID IN THE ANODE SECTION OF AN ELECTROLYSIS CELL HAVING AN ANION EXCHANGE MEMBRANE SEPARATING THE ANODE SECTION FROM THE CATHODE SECTION, SIMULTANEOUSLY ELECTROLYZING PREGNANT LEACH LIQUOR WHICH CONTAINS PHOSPHATE ION IN THE CATHODE SECTION, CONTINUING THE ELECTROLYSIS UNTIL THE URANIUM IS PRECIPITATED AS URANOUS PHOSPHATE, SEPARATING THE URANOUS PHOSPHATE 